In cellular communication systems, various techniques for enhancing any one of bandwidth, throughput, system coverage and performance are studied in view of respective limitations inherent to conventional operations of cellular communications. In this regard, device-to-device (D2D) communication is developed for providing a D2D communication mode or service between a pair of devices (such as terminals or user equipment operable in a cellular communication system like a LTE/LTE-A system), in which at least a core network of the communication system is not involved, in addition to a conventional communication mode or service (also referred to as infrastructure mode or service) between devices (such as terminals or user equipment operable in a cellular communication system like a LTE/LTE-A system) and the core network of the communication system (such as an EPC of a LTE/LTE-A system).
Such D2D communication is generally applicable to any pair of devices in mutual proximity, which is why the D2D communication mode or service may also be referred as proximity mode or service. Examples of D2D communication may include direct communications in a cluster of proximity devices, D2D communications in a cellular network, or the like. A communication (data) path for such D2D communication may be arranged directly between the involved devices, which is referred to as direct path or mode, or may be arranged via at least one serving base station of the cellular communication system, which is referred to as locally-routed/optimized path or mode.
Typically, such D2D communication is subject to network control, thus being referred to as network- or operator-controlled D2D communication. For example, the devices involved in a D2D communication may be controlled in terms of resource management and/or connection management by network entities, i.e. radio access network entities such as a base station (e.g. an eNB of a LTE/LTE-A system) and core network entities such as a mobility management entity (MME) and/or a D2D registration server function (DRSF).
For D2D communication, both under optimized path and direct path, it is adopted that radio resources for the involved pair of devices should be controlled/managed by way of the RRC layer, i.e. via a RRC connection, so as to enable execution of (D2D) radio bearer setup/modification/release procedures and the like.
In current cellular communication systems, a RRC layer is defined, i.e. a RRC connection between a terminal or user equipment and a base station is established, which is in charge of controlling/managing radio resources between the terminal or user equipment and the network side for core network communication. For example, in LTE/LTE-A systems, a RRC layer is defined between UE and eNB for controlling/managing radio resources for conventional EPS services and/or NAS services. Herein, both EPS services (i.e. user-plane services, such as e.g. voice or data services used by a user) and NAS services (i.e. control plane services, such as e.g. a tracking area update) may be commonly referred to as core network (CN) communication services. Such RRC layer/connection is established to support core network communication such as EPS bearer transmission and/or NAS PDU transmission between the core network and the UE, which means that the RRC layer/connection in a LTE/LTE-A system will only exist when it is associated with a bearer towards the core network side, e.g. a S1 bearer or a S5 bearer. Stated in other words, there will be at least an EPS bearer and/or NAS signaling connection existing between the core network and the UE for a typical RRC_Connected state according to current specifications, which means that the RRC_Connected state is combined with an existing EPS service and/or NAS service, i.e. the RRC connection provides a tunnel for upper layer data transmission e.g. NAS PDUs towards the core network. Accordingly, any CN communication service (i.e. any EPS and/or NAS service) conventionally requires existence/availability of both a RRC layer/connection and a S1 bearer.
For D2D communication, both under optimized path and direct path, there is no requirement or restriction that a core network communication is established, i.e. an EPS service or bearer transmissions and/or a NAS service or signaling connection is active. In the case that no core network communication is established, i.e. no EPS service or bearer transmission and no NAS service or signaling connection is active, no RRC layer/connection is established either. Stated in other words, since a typical RRC_Connected state according to current specifications is combined with an existing EPS service and/or NAS service, the related RRC connection to certain devices will be released (i.e. the RRC state will be set to RRC_Idle), once no EPS service and no NAS service is active, i.e. once no dedicated EPS bearers and/or NAS signaling connections for the certain devices are available. In the absence of a RRC connection to devices involved in a D2D communication, i.e. with these devices being set in the RRC_Idle state, due to the lack of an EPS service and/or NAS service for these devices, no radio resource control/management for a D2D communication of these devices can thus be handled by way of RRC according to current specifications, namely RRC in charge of handling radio resource control/management for core network communication services.
That is, management of radio resources for D2D communication becomes an issue, since the RRC layer/connection will be released immediately when the combined core network communication service (i.e. EPS service and/or NAS service) is over. In view thereof, an RRC layer/connection could not be utilized for radio resource control/management for D2D communication in a reliable manner.
Therefore, there resides a problem in how to manage radio resource, or how to manage radio resource control states, for D2D communication to support such radio resource management without a requirement or restriction that a core network communication needs to be established for at least one device involved in the D2D communication.
Accordingly, there is a demand for an appropriate management of radio resource control for device-to-device communication.